Ian Kennedy came to the Diamondbacks in a 2009 trade with the Yankees. / Kim Klement, USA TODAY Sports

by Bob Nightengale, USA TODAY Sports

by Bob Nightengale, USA TODAY Sports

The Arizona Diamondbacks traded Ian Kennedy, who won 48 games for them over four seasons, to the San Diego Padres in a deal that fortifies their bullpen but deletes a starting pitcher from their rotation.

San Diego will send reliever Joe Thatcher, Class AA reliever Matt Stites and a compensatory-round draft pick to the Diamondbacks, according to a baseball official with direct knowledge of the trade. The official spoke to USA TODAY Sports on condition of anonymity because the trade has not been finalized.

In a unique move of a key player within the same division, the Diamondbacks shed a player who will be arbitration-eligible this off-season in exchange for a reliever that general manager Kevin Towers also acquired when he was San Diego's GM.

Though Kennedy, 28, has been Arizona's ace at times in his four seasons there - winning 21 games in 2011 - Towers has not shied away from dealing players once seen as core assets.

And Kennedy has regressed since 2011, winning 15 games last year but seeing his earned-run average spike from 2.88 to 4.02. This season, he's 3-8 with a 5.23 ERA, and lasted five innings in his final game with Arizona, a 6-3 loss to the Tampa Bay Rays on Tuesday night.

He's making $4.27 million this season and will be due another raise after this year. The D'backs were considering not offering him a contract this winter, and with Trevor Cahill and Brandon McCarthy returning from injuries, Kennedy was expendable. Arizona entered Wednesday's games 3½ behind the Los Angeles Dodgers in the NL West.

Kennedy not eligible for free agency until after the 2015 season, and could benefit from the friendlier conditions for pitchers at San Diego's Petco Park.